Add documentation warnings for weak algorithms

MD2, MD4, MD5, DES and SHA-1 are considered weak and their use
constitutes a security risk. If possible, we recommend avoiding
dependencies on them, and considering stronger message digests and
ciphers instead.

include/mbedtls/des.h

@@ -2,6 +2,10 @@
  * \file des.h
  *
  * \brief DES block cipher
+ *
+ * \warning   DES is considered a weak cipher and its use constitutes a
+ *            security risk. We recommend considering stronger ciphers
+ *            instead.
  */
 /*
  *  Copyright (C) 2006-2015, ARM Limited, All Rights Reserved
@@ -20,6 +24,7 @@
  *  limitations under the License.
  *
  *  This file is part of mbed TLS (https://tls.mbed.org)
+ *
  */
 #ifndef MBEDTLS_DES_H
 #define MBEDTLS_DES_H
@@ -51,6 +56,10 @@ extern "C" {
 
 /**
  * \brief          DES context structure
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 typedef struct
 {
@@ -71,6 +80,10 @@ mbedtls_des3_context;
  * \brief          Initialize DES context
  *
  * \param ctx      DES context to be initialized
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 void mbedtls_des_init( mbedtls_des_context *ctx );
 
@@ -78,6 +91,10 @@ void mbedtls_des_init( mbedtls_des_context *ctx );
  * \brief          Clear DES context
  *
  * \param ctx      DES context to be cleared
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 void mbedtls_des_free( mbedtls_des_context *ctx );
 
@@ -102,6 +119,10 @@ void mbedtls_des3_free( mbedtls_des3_context *ctx );
  *                 a parity bit to allow verification.
  *
  * \param key      8-byte secret key
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 void mbedtls_des_key_set_parity( unsigned char key[MBEDTLS_DES_KEY_SIZE] );
 
@@ -114,6 +135,10 @@ void mbedtls_des_key_set_parity( unsigned char key[MBEDTLS_DES_KEY_SIZE] );
  * \param key      8-byte secret key
  *
  * \return         0 is parity was ok, 1 if parity was not correct.
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 int mbedtls_des_key_check_key_parity( const unsigned char key[MBEDTLS_DES_KEY_SIZE] );
 
@@ -123,6 +148,10 @@ int mbedtls_des_key_check_key_parity( const unsigned char key[MBEDTLS_DES_KEY_SI
  * \param key      8-byte secret key
  *
  * \return         0 if no weak key was found, 1 if a weak key was identified.
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 int mbedtls_des_key_check_weak( const unsigned char key[MBEDTLS_DES_KEY_SIZE] );
 
@@ -133,6 +162,10 @@ int mbedtls_des_key_check_weak( const unsigned char key[MBEDTLS_DES_KEY_SIZE] );
  * \param key      8-byte secret key
  *
  * \return         0
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 int mbedtls_des_setkey_enc( mbedtls_des_context *ctx, const unsigned char key[MBEDTLS_DES_KEY_SIZE] );
 
@@ -143,6 +176,10 @@ int mbedtls_des_setkey_enc( mbedtls_des_context *ctx, const unsigned char key[MB
  * \param key      8-byte secret key
  *
  * \return         0
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 int mbedtls_des_setkey_dec( mbedtls_des_context *ctx, const unsigned char key[MBEDTLS_DES_KEY_SIZE] );
 
@@ -198,6 +235,10 @@ int mbedtls_des3_set3key_dec( mbedtls_des3_context *ctx,
  * \param output   64-bit output block
  *
  * \return         0 if successful
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 int mbedtls_des_crypt_ecb( mbedtls_des_context *ctx,
                     const unsigned char input[8],
@@ -221,6 +262,10 @@ int mbedtls_des_crypt_ecb( mbedtls_des_context *ctx,
  * \param iv       initialization vector (updated after use)
  * \param input    buffer holding the input data
  * \param output   buffer holding the output data
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 int mbedtls_des_crypt_cbc( mbedtls_des_context *ctx,
                     int mode,
@@ -279,6 +324,10 @@ int mbedtls_des3_crypt_cbc( mbedtls_des3_context *ctx,
  *
  * \param SK       Round keys
  * \param key      Base key
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 void mbedtls_des_setkey( uint32_t SK[32],
                          const unsigned char key[MBEDTLS_DES_KEY_SIZE] );